I think in the famous case where a Boeing 727 did some aerobatics at high altitude the guys erased the CVR after landing. The FAA were therefore unable to prove that they had committed any errors which caused the loss of control but nevertheless insisted that they had, according to other evidence. SEE TWA Flight 841 (1979)

The CVR was not damaged; however, 21 .minutes of the 30-minute tape were blank. The remaining 9 minutes of tape were of good fidelity, but they pertained only to flightcrew conversations after the aircraft was on the ground at Detroit. (See appendix D.) Tests of the CVR in the aircraft revealed no discrepancies in the CVR’s electrical and recording systems. The CVR tape can be erased by means of the bulk-erase feature on the CVR control panel located in the cockpit. This feature can be activated only after the aircraft is on the ground with its parking brake engaged. In a deposition taken by the Safety Board, the captain stated that he usually activates the bulk-erase feature on the CVR at the conclusion of each flight to preclude inappropriate use of recorded conversations. However, in this instance, he could not recall having done so. The first and second officers both stated that they did not erase the tape nor did they see the captain activate the erase button on the CVR control panel.